and even though this has been used, this type of technology has been used by al qaeda and taliban in the past, it is also sadly in this age of the internet a technology you can go online and essentially find a how-to to build a bomb like this. no one has ruled out domestic terrorism, or this being a single lone wolf bomber, perhaps someone with help. right now, they say those are among the many, many questions. >> and it's significant also that no one has claimed responsibility. there's almost a thunderous silence, if you will. that speaks volumes to a lot of these investigators. >> if you look past backwards, past terrorist attacks, obviously if it's al qaeda, if it's al qaeda an the arabian peninsula, even a domestic political group trying to make a point, often there's a claim of responsibility in the first day or two. the silence is part of the investigation, and a reminder to us that what is not being said and what we don't know is perhaps much more important than what we do know. >> john, don't go too far away. moments before the terror attack here in boston, 29-year-old krystl